Characterization of novel genes regulated by PPARgamma ligands in 3T3-L1 adipocytes

The nuclear hormone receptor PPARgamma is the master transcriptional regulator of adipogenesis. Thiazolidinediones (TZDs), which improve insulin sensitivity in type 2 diabetes, are high-affinity ligands for PPARgamma and are believed to mediate their effects through this receptor. Paradoxically, TZDs can also activate the adipogenic gene program and cause weight gain and increased adiposity, a state generally associated with insulin resistance. We hypothesize that there must exist a subset of PPARgamma-regulated genes in the adipocyte that is differentially regulated between the adipogenic and the antidiabetic gene programs upon treatment with PPARgamma ligands, and that identifying such genes is critical towards understanding how these ligands improve insulin sensitivity. Using microarray analysis performed on differentiated 3T3-L1 mouse adipocytes treated with various PPARgamma ligands, a large number of novel candidate genes were identified. Two such genes, activating transcription factor 3 (ATF3) and PPARgamma coactivator-1 alpha (PGC-1alpha), were confirmed to be upregulated by PPARgamma ligand treatment, and initial characterization of their expression patterns in adipocytes was performed. The oxidized low-density lipoprotein receptor 1 (OLR1) was also shown to be dramatically upregulated by TZDs in adipocytes. OLR1 is a direct PPARgamma target gene that contains four distinct PPAR response elements in its promoter, and is transcriptionally regulated by the exchange of corepressors for coactivators in the presence of a PPARgamma ligand. TZDs markedly stimulate the uptake of oxLDL into adipocytes, in part through the induction of OLR1. Increasing OLR1 expression, either by TZD treatment or by adenoviral gene delivery, significantly augments adipocyte cholesterol content and enhances free fatty acid uptake. The identification of OLR1 has uncovered not only a potential new role for PPARgamma in the coordination of cholesterol and fatty acid metabolism in adipocytes, but also a novel mechanism by which TZDs may work. Further functional characterization of genes identified from the microarray studies will offer new insights towards the elucidation of novel pathways involved in PPARgamma-mediated insulin sensitization.